Warpage

Warpage is the part losing its shape as a result of different areas shrinking at different rates. A part that looks straight when it comes out of the mold can bend within hours. At its root there is almost always an imbalance: a temperature difference between the mold halves, a difference in wall thickness, or a difference in the orientation of the aligned molecules and fibers.

How to Recognize It

• The part deviates from its shape after coming out of the mold (sometimes within hours); in box parts the edge bends inward/outward.
• In glass-fiber parts, systematic warpage depending on the flow direction is typical.

Physical Causes

• Warpage is ALWAYS the result of differences in shrinkage: areas cool at different rates and shrink differently.
• The two halves of the mold are at different temperatures; differences in wall thickness (a thick area shrinks a lot).
• Semi-crystalline materials shrink much more than amorphous ones; glass fiber orientation makes shrinkage direction-dependent (fiber:plastic shrinkage 1:200).
• A low mold temperature reduces mold shrinkage but increases the FINAL shrinkage (post-shrinkage) — the part warps in the field.

Solution Steps at the Machine

1. Measure the temperature of the two halves of the mold and balance/set them separately with the temperature control unit — the single most effective step.
2. Increase the holding pressure and time (reduces shrinkage); balance locally by profiling.
3. The criterion is not the thickness itself but the ratio FLOW PATH ÷ WALL THICKNESS: if the ratio is large (thin wall, long flow), keep the melt temperature and injection speed HIGH; if the ratio is small (thick wall, short flow), keep them LOW. Raising them improves flow and the transfer of holding pressure, but also increases the shrinkage potential; the right side is chosen by looking at this ratio.
4. Increase the cooling time; cool the part on a flat surface.

Mold, Design and Material Solutions

• Uniform wall thickness; avoid sudden transitions and melt accumulations; symmetrical design.
• Move the gate to the thickest area (the wedge principle — the melt should flow from thick to thin; the reverse cuts off the transfer of holding pressure); provide a uniform flow direction through the number/position of gates; avoid weld lines.
• Set up separate temperature control circuits in the mold; thermally separate the part from the hot runner nozzle.
• Consider low-warpage materials or mineral/glass-bead filled grades (combined with fiber).

Important Notes

• ⚠️ Shrinkage is divided into two: MOLD SHRINKAGE (the dimensional change 16 hours after ejection from the mold according to DIN 16901) and POST-SHRINKAGE (the change that continues over a long period after that point). If the measurement is made before 16 hours have passed, the part is still shrinking; do not decide on a mold correction with this measurement.

Which defect is it confused with?

A wrong diagnosis leads to turning the right parameter in the wrong direction. This defect can be confused with the ones below; the right-hand column gives the practical way to tell them apart.

Frequently asked questions